Physical Modeling of the downwash effect of rooftop structures on plume dispersion

One of the major causes of poor indoor air quality in buildings is exhaust re-ingestion at fresh air intakes. The downwash effect of an RTS on plume may lead to significantly increased levels of re-ingestion. The present study aims to quantify the downwash effect of an RTS located upwind of a stack. Wind-tunnel modeling is used to model the flow and dispersion associated with various building/RTS arrangements with a focus on "micro-scale" urban dispersion; that is, modeling dispersion within a range of 100 m of the building exhaust. Wind-tunnel techniques include flow visualization and tracer gas experiments. The influence of various key parameters, such as building height, RTS crosswind width, stack height, exhaust momentum ratio, stack location, and wind direction is considered. Concentration measurements were obtained at sampling locations downwind of the RTS on the building roof. It was found that the downwash effect of RTS on plumes could increase roof -level concentrations by a factor of 2 to 100. The RTS downwash effect was intensified with an increase in RTS crosswind width and was generally stronger for an oblique wind than for a normal wind. The results also showed that an RTS engulfed inside the building recirculation zone may not have a significant effect on plume dispersion for winds approximately normal to the building face. The minimum dilution models recommended by ASHRAE (2003, 2007) as well as the ASHRAE Geometric Stack Design Method (AGM) were evaluated. It was found that the ASHRAE models do not account for the downwash effect of the RTS on plumes and result in either overly conservative or un-conservative predictions. Therefore, a new empirical model, which takes into consideration the downwash effect of RTS was proposed. A validation study of the proposed model using results from previous experimental studies showed that the model is more accurate than the ASHRAE dilution models. Thus the proposed model will be very useful to practicing engineers. In addition, some modifications to the ASHRAE (2007) dispersion model and the AGM are also proposed

[1]  I. V. D. Hoven.  POWER SPECTRUM OF HORIZONTAL WIND SPEED IN THE FREQUENCY RANGE FROM 0.0007 TO 900 CYCLES PER HOUR , 1957 .

[2]  J. Halitsky.  Diffusion of Vented GAS Around Buildings , 1962 .

[3]  Alan H. Huber.  Wind tunnel and Gaussian plume modeling of building wake dispersion , 1991 .

[4]  A. Robins,et al.  The flow around a surface-mounted cube in uniform and turbulent streams , 1977, Journal of Fluid Mechanics.

[5]  J. Counihan,et al.  An improved method of simulating an atmospheric boundary layer in a wind tunnel , 1969 .

[6]  Theodore Stathopoulos,et al.  Dilution of exhaust from a rooftop stack on a cubical building in an urban environment , 2002 .

[7]  J. Cermák.  Physical Modelling of Flow and Dispersion over Urban Areas , 1995 .

[8]  J. Hunt,et al.  Kinematical studies of the flows around free or surface-mounted obstacles; applying topology to flow visualization , 1978, Journal of Fluid Mechanics.

[9]  Robert N. Meroney,et al.  Bluff-body aerodynamics influence on transport and diffusion , 1990 .

[10]  R. P. Hosker,et al.  Flow around isolated structures and building clusters: A review , 1985 .

[11]  B K Lamb,et al.  Fume hood exhaust re-entry into a chemistry building. , 1986, American Industrial Hygiene Association journal.

[12]  J. V. Ramsdell,et al.  Estimating concentrations in plumes released in the vicinity of buildings: model development , 1998 .

[13]  F. B. Smith,et al.  UK-ADMS: A new approach to modelling dispersion in the earth's atmospheric boundary layer , 1994 .

[14]  Elisa Canepa,et al.  An overview about the study of downwash effects on dispersion of airborne pollutants , 2004, Environ. Model. Softw..

[15]  E. H. Chui,et al.  Influence of building size on rooftop dispersion of exhaust gas , 1994 .

[16]  Ronald L. Petersen.  Wind Tunnel Investigation of the Effect of Platform-Type Structures on Dispersion of Effluents from Short Stacks , 1986 .

[17]  Jon A. Peterka,et al.  Wind flow patterns about buildings , 1985 .

[18]  Alan H. Huber.  Performance of a Gaussian model for centerline concentrations in the wake of buildings , 1988 .

[19]  Theodore Stathopoulos,et al.  The influence of freestream turbulence on nearfield dilution of exhaust from building vents , 1998 .

[20]  D. Wilson,et al.  Interaction of a roof-level plume with a downwind building , 1978 .

[21]  Kiyoshi Uehara,et al.  Field and wind tunnel study of the flow and diffusion around a model cube—I. Flow measurements , 1983 .

[22]  An examination of simple models for building influenced dispersion , 1984 .

[23]  A. G. Davenport,et al.  The relationship of wind structure to wind loading , 1966 .

[24]  Christopher D. Jones,et al.  Effect of atmospheric stability on concentration fluctuations and wake retention times for dispersion in the vicinity of an isolated building , 1995 .

[25]  William H. Snyder,et al.  Wind tunnel investigation of the effects of a rectangular-shaped building on dispersion of effluents from short adjacent stacks , 1982 .

[26]  Theodore Stathopoulos,et al.  Effects of model scale in estimating pollutant dispersion near buildings , 1995 .

[27]  M. Fulker,et al.  A wind-tunnel examination of the variation of sigma Y and sigma Z with selected parameters , 1994 .

[28]  K. P. Lowery,et al.  A Wind Tunnel Study into the Effects of Raised Intakes and Parapets on Fresh Air Intake Contamination by a Rooftop Stack. , 1996, Journal of the Air & Waste Management Association.

[29]  S. Oikawa,et al.  A field study of diffusion around a model cube in a suburban area , 1997 .

[30]  John Pye,et al.  Review and Evaluation of Models Estimating the Minimum Atmospheric Dilution of Gases Exhausted Near Buildings , 1995 .

[31]  William H. Snyder,et al.  Determination of a necessary height for a stack close to a building—a wind tunnel study , 1976 .

[32]  W. H. Snyder,et al.  Wind-tunnel measurements of flow fields in the vicinity of buildings , 1994 .

[33]  J. D. Holmes,et al.  Wind Loading of Structures , 2001 .

[34]  Louis Lazure,et al.  Contamination of Fresh Air Intakes Due to Downwash from a Rooftop Structure , 2009, Journal of the Air & Waste Management Association.

[35]  R. L. Petersen,et al.  Influence of Architectural Screens on Rooftop Concentrations Due to Effluent from Short Stacks , 1999 .

[36]  Brian K. Lamb,et al.  Dispersion of exhaust gases from roof-level stacks and vents on a laboratory building , 1994 .

[37]  William H. Snyder.  Downwash of Plumes in the Vicinity of Buildings: A Wind-Tunnel Study , 1994 .

[38]  Erich J. Plate,et al.  Wind field and dispersion in a built-up area—A comparison between field measurements and wind tunnel data , 1991 .

[39]  Christopher D. Jones,et al.  Wind tunnel investigation of dispersion of pollutants due to wind flow around a small building , 1994 .

[40]  Roger S. Thompson.  Building amplification factors for sources near buildings: A wind-tunnel study , 1993 .

[41]  L L Schulman,et al.  Development and Evaluation of the PRIME Plume Rise and Building Downwash Model , 2000, Journal of the Air & Waste Management Association.

[42]  J. V. Ramsdell,et al.  Diffusion in building wakes for ground-level releases , 1990 .

[43]  D. Wilson,et al.  Influence of exhaust velocity and wind incidence angle on dilution from roof vents , 1985 .

[44]  D. J. Hall,et al.  Concentration measurements around an isolated building: A comparison between wind tunnel and field data , 1994 .

[45]  Rex Britter,et al.  Estimates of building surface concentrations from nearby point sources , 1982 .